1. Field of the Invention
The present invention relates to a retardation film or a phase difference film prepared by irradiating a layer of a mixture of a photosensitive polymer and a low molecular weight compound with linear polarized ultraviolet rays, ultraviolet rays including a perfectly polarized light component and non-polarized light component or non-polarized ultraviolet rays (hereinafter referred to as exposure according to the need) to give molecular orientation thereby developing a retardation and the direction of the optical axis optionally in the above layer of the mixture, and to a process for producing the retardation film.
2. Description of the Prior Art
The retardation films are films passing linear polarized light components oscillating in the major axis directions orthogonal to each other and having birefringence providing a given retardation between these two components. Such retardation films are also utilized in the fields of liquid crystal displays. Particularly retardation films, of which the optical axis is inclined against a principal surface of the film, specifically, the optical axis is inclined with the principal surface of the film and its normal line, serve to enlarge a viewing angle or angle of visibility of a liquid crystal display as optical compensation films decreasing the visual field dependency of the transmission quantity of light which is caused by the inclination of the molecular arrangement of a liquid crystal cell.
There are several prior art technologies for producing such retardation films.
As one of these technologies, there is a process in which a polymer material such as polycarbonate is stretched to orient a high molecular chain thereby producing a difference between the refractive index in the stretching direction and the refractive index in the direction orthogonal to the stretching direction. However, it is difficult to incline the optical axis against the principal surface of the film by this stretching method because molecules are oriented in the stretching direction.
Also, as a method for developing a retardation by exposure to polarized light, there is a method in which a photosensitive polymer such as polyvinyl cinnamate is irradiated with polarized UV light (JP A-7-138308). However, in this method, anisotropy is developed in a direction orthogonal to the direction of the electric field oscillation of the applied polarized UV light and the optical axis cannot be therefore inclined, so that a viewing angle is enlarged with difficulty.
To solve the foregoing problem, a method is known in which a liquid crystal monomer is aligned and fixed on an alignment layer obtained by irradiation with polarized UV light, rubbing treatment or slating deposition of SiO (JP A-8-15681). A method is also known in which a discotic liquid crystal is arranged on an alignment layer prepared by rubbing or an alignment layer prepared by SiO slanting deposition (each of JP A-7-287119 and JP A-7-287120). Further, a method is known in which a discotic liquid crystal containing a photo polymerization initiator is aligned on a photo alignment layer and this alignment is fixed by irradiation with light (JP A-10-278123). However, in these methods using an alignment layer, the process involving the aligning treatment of the alignment layer and the alignment of a liquid crystal material are complicated, raising the production cost of a retardation film which has a large area and inclined optical axis.
Moreover, as one of other processes for producing a retardation film having inclined optical axis, a method is proposed in which an inorganic dielectric is slant-deposited. This method poses the problems that a large scale apparatus is required to form a deposited film continuously on a lengthy sheet and that the process is complicated. The inventor of the present invention has proposed a process for producing a retardation film with inclined optical axis by exposing a side chain liquid crystalline polymer having photo-sensitivity to polarized light (JP A-10-278123). However, this method develops a large retardation, giving rise to the problems that the cloudiness is increased as the film is thickened.
The invention provides a retardation film suitable for mass production using a simple process and a process for producing the film.
According to a first aspect of the present invention, there is provided a process for producing a retardation film, the method comprising producing a retardation film by a process including a step of irradiating a mixture of a photosensitive polymer and a low molecular weight compound with light, wherein the ratio z of solubility parameters calculated from the evaporation energy and molecular volume of these photosensitive polymer and low molecular weight compound is as follows: 0.93 less than z less than 1.06.
In this production process, the mixture of the photosensitive polymer and low molecular weight compound which are compatible is formed in film and exposed to light, whereby the photosensitive polymer and the low molecular weight compound can be oriented. When this exposure is conducted from a direction inclined with a principal surface of the layer formed of the aforementioned mixture, the layer can be oriented with its optical axis being arbitrarily inclined. Therefore, a retardation film with optical axis inclined in a desired direction is obtained.
The low molecular weight compound has the effect of suppressing cloudiness as far as its amount is proper, but on the contrary, causes an increase in cloudiness and a reduction in orientation property when the amount to be added is excessive. From this point of view, the amount of the low molecular weight compound is preferably 5 wt % to 50 wt % though it depends on the types of the photosensitive polymer or low molecular weight compound and the retardation film can be produced if the low molecular weight compound is added in the amount of 0.1 wt % to 80 wt %.
Here, when the compatibility between the photosensitive polymer and the low molecular weight compound is insufficient, a crystal with a size enough to induce phase separation and scattering of visible light by heating a substrate during film forming or after exposure is generated, causing an increase in cloudiness. In order to suppress the phase separation and the generation of microcrystals, it is necessary to control the compatibility between the polymer and the low molecular weight compound.
As a standard of this compatibility, the solubility parameter ("sgr") calculated from evaporation energy (xcex94Ev) and molecular volume (V) according to the calculation formula (1) as described in Polymer Engineering and Science, Vol. 7, No.2, 147 (1974) may be utilized for the sake of convenience. It has been found from the result of an experiment that when the ratio z of the solubility parameter ("sgr") of the polymer to the solubility parameter ("sgr") of the low molecular weight compound is in the range of 0.93 less than z less than 1.06, the phase separation and the generation of microcrystals are efficiently suppressed, whereby the cloudiness of the resulting retardation film can be decreased.
"sgr"=(xcex94EV/V)xc2xdxe2x80x83xe2x80x83(1)
The adoption of the aforementioned production process enables mass production of a retardation film in a simple process. Also, the resulting retardation film has decreased cloudiness and is superior in the effect of enlarging a viewing angle when it is used for a liquid crystal display.
As the above photosensitive polymer to be used as the raw material of the retardation film, a liquid crystal material is preferably used.
In addition, as the photosensitive polymer, those preferably used are photosensitive homopolymers or copolymers which have at least one of structures represented by the following molecular structure 1 to 9 and in which the main chain represented by the molecular structure 10 is a hydrocarbon, acrylate, methacrylate, maleimide, N-phenylmaleimide or siloxane. Also, as the low molecular weight compound, those having a molecular structure represented by the molecular structure 11 or 12 are preferable. 
wherein xe2x80x94R1 to xe2x80x94R11=xe2x80x94H, halogen group, xe2x80x94CN, alkyl group or alkyloxy group, e.g., methoxy group or group obtained by fluorinating each of these groups, xe2x80x94R12=alkyl group, e.g., methyl group or ethyl group or group obtained by fluorinating each of these groups, x:y=100 to 0:0 to 100, n=1 to 12, m=1 to 12, j=1 to 12, o=1 to 12, p=1 to 12, q=1 to 12, X, Y=none, xe2x80x94COO, xe2x80x94OCOxe2x80x94, xe2x80x94Nxe2x95x90Nxe2x80x94, xe2x80x94Cxe2x95x90Cxe2x80x94 or xe2x80x94C6H4xe2x80x94, W1, W2, W3, W4, W5, W6=structure represented by molecular structure 1, 2, 3, 4, 5, 6, 7, 8 or 9.
As the light with which the above mixture of the photosensitive polymer and the low molecular weight compound is irradiated, linear polarized light or light including a perfectly polarized light component and a non-polarized light component is preferably used. Also, the above light is preferably irradiated on both of opposite principal surfaces of the above mixture. Further, when producing the retardation film, it is also preferable to involve a heating step and/or a cooling step. Also, in the production of the retardation film, it is also preferable to crosslink the photosensitive polymer or low molecular weight compound constituting the film.
The retardation film of this invention is produced by the aforementioned production process according to the first aspect. The retardation film obtained in this manner has decreased cloudiness and is superior in the effect of enlarging a viewing angle when it is used for a liquid crystal display.
According to a second aspect of the present invention, there is provided a process for producing a retardation film, the method comprising irradiating a layer formed of a photosensitive polymer or a mixture of a photosensitive polymer and a low molecular weight compound with light while changing the incident angle of the light in regular sequence.
This production process according to the second aspect of the present invention enables the production of a retardation film (optically anisotropic element) effective to enlarge the viewing angle of a liquid crystal display in which the inclination of each optical axis of index ellipsoids (side chain portion of the photosensitive polymer) is no uniform in a simple process and also enables mass production.
In the aforementioned production process according to the second aspect of the present invention, it is preferable to irradiate the layer formed of the aforementioned photosensitive polymer or mixture of the photosensitive polymer and the low molecular weight compound in both directions of incoming to the opposite principal surfaces with the incident angle changing in sequence. Also, it is also preferable to include a heating step and/or a cooling step. It is also preferable to crosslink the photosensitive polymer or low molecular weight compound constituting the retardation film.
It is preferable to add a uniaxial index ellipsoid (side chain portion of the photosensitive polymer) layer or/and a biaxial index ellipsoid (side chain portion of the photosensitive polymer) layer to the aforementioned retardation film (optically anisotropic element). This structure ensures that a retardation film having a higher effect of enlarging a viewing angle when the film is used for a liquid crystal display is produced.
As the above photosensitive polymer to be used as the raw material of the above retardation film (optical anisotropic element), a liquid crystal material is preferably used.
According to a third aspect of the present invention, there is provided a process for producing a retardation film, the method comprising irradiating a layer formed of a photosensitive polymer or a mixture of a photosensitive polymer and a low molecular weight compound with linear polarized lights having electric field oscillation planes differing from each other in at least two directions, wherein the light irradiated at this time in at least one direction is irradiated in a direction oblique to the normal line of the aforementioned layer.
In this production process according to the third aspect of this invention, a layer which is oriented with the optical axis being arbitrarily inclined can be formed. A retardation film (optically anisotropic element) effective to enlarge the viewing angle of a liquid crystal display even if only one layer is used can be obtained in a simple process and also mass production is made possible.
The aforementioned process of irradiating with linear polarized light includes, for example, a step of irradiating the layer with a first linear polarized light in a direction oblique to the normal line of the layer and a step of irradiating the layer with a second linear polarized light whose electric field oscillation plane is on the same plane as that of the first light in the direction of the normal line of the layer. The aforementioned process of irradiating with light may include a step of irradiating the layer with a first linear polarized light in a direction oblique to the direction of the normal line of the layer and a step of irradiating the layer with a second linear polarized light whose electric field oscillation plane is orthogonal to that of the first linear polarized light in the direction of the normal line of the layer. Also, in the aforementioned process of irradiating with light, the layer may be irradiated with linear polarized lights having electric field oscillation planes orthogonal to each other in two directions oblique to the direction of the normal line of the layer.
In the production process according to the third aspect of the present invention, the irradiation of the layer formed of the photosensitive polymer or the mixture of the photosensitive polymer and the low molecular weight compound with linear polarized light is preferably conducted on both of opposite principal surfaces, or on the two principal surfaces, of the layer. Also, the method preferably comprises a heating step and/or a cooling step. Further, it is also preferable to crosslink the photosensitive polymer or low molecular weight compound constituting the retardation film.
In addition, in the retardation film, two or more layers are preferably laminated in such an arrangement as to make respective optical anisotropic axes orthogonal or perpendicular to each other.
According to a fourth aspect of the present invention, there is provided a process for producing a retardation film, the method comprising irradiating a layer made of a photosensitive polymer containing a positive index ellipsoid structure or a mixture of the polymer and a low molecular weight compound with non-polarized light or light including a perfectly polarized light component and a non-polarized light component, to thereby control birefringence.
In the production process according to the fourth aspect of the present invention, a retardation film effective to enlarge the viewing angle of a liquid crystal display can be obtained in a simple process and also mass production is made possible by controlling the birefringence.
In the production process according to the fourth aspect of the present invention, it is preferable to control the ratio of three primary refractive indexes nx, ny and nz of the index ellipsoid in the film and the inclination of the nx axis with the direction of the normal line of the film surface by irradiating the layer made of the photosensitive polymer containing the positive index ellipsoid structure or the mixture of the polymer and the low molecular weight compound with non-polarized light or light including a perfectly polarized light component and a non-polarized light component. Here, the term xe2x80x9ccontrolxe2x80x9d is, specifically, to develop the same birefringence as in the case of combining a slant-oriented index ellipsoid, bend-oriented index ellipsoid or non-slanted uniaxial index ellipsoid.
The aforementioned production process according to the fourth aspect of the present invention preferably comprises a step of heating and/or cooling the above layer. The method also preferably comprises a step of irradiating the layer made of the photosensitive polymer containing the positive index ellipsoid structure or the mixture of the polymer and the low molecular weight compound with non-polarized light or light including a perfectly polarized light component and a non-polarized light component in both directions of the surface and backface of the layer. Further, it is also preferable to irradiate the layer made of the photosensitive polymer containing the positive index ellipsoid structure or the mixture of the polymer and the low molecular weight compound with non-polarized light or light including a perfectly polarized light component and a non-polarized light component in a direction oblique to the direction of the normal line of the layer surface.
As the photosensitive polymer which is the raw material of the aforementioned retardation film, a liquid crystal material is preferably used. Also, as the low molecular weight compound which is the raw material of the retardation film, a crystalline or liquid crystal material is preferably used. Further, this low molecular weight compound preferably has a reactive group which is crosslinked or polymerized by light or heat. Also, when the above retardation film is produced, a step of crosslinking the photosensitive polymer and/or the low molecular weight compound is preferably involved.
The retardation film (optically anisotropic element) of the present invention which is produced by any one of the aforementioned second to fifth production process has a superior effect of enlarging a viewing angle when it is used for a liquid crystal display.